Celestia gestured with a forehoof. "I am Princess Celestia. To my left sits Luna. To my right, Cadance." Celestia frowned. "But one of these things I say is not true."
Twilight blinked. "Not true?"
"Not true. My little student, can you tell me which one?"
Twilight trotted closer, to carefully examine the suspects. Twilight said, "Sunshine!" Cadance and Twilight danced, reciting a rhyme about ladybugs. Twilight cast a barrage of spells at each supposed alicorn one by one, and at all of the alicorns together.
After her final pyrotechnics, Twilight panted with effort. "I don't know. I don't know! Where's the falsehood?"
Celestia smiled. "One of these things I say is not true."
"That was the lie?"
Celestia nodded.
"But that's a paradox! It's neither true NOR false!" Twilight snorted scornfully. "The Celestia I know would never cheat at a game. Unforgivable!"
Twilight threw several fireballs. "Look, I found Chrysalis!"